Mass concrete underpinning: the traditional method

How mass concrete underpinning works

Mass concrete underpinning is exactly what it sounds like. The contractor excavates a small pit beside the wall, then digs under the existing footing to a depth the engineer has specified — down to soil that can genuinely carry the house. That void is filled with concrete and left to cure. Because wet concrete shrinks slightly as it sets, a gap of around 50 mm is left between the top of the new block and the underside of the old footing, and this is "dry-packed": a stiff sand-and-cement mortar is rammed in hard so the footing bears on its new base with no slack at all.

The work proceeds in short sections called pins, usually about a metre wide. One pin is dug, poured, cured and packed before its neighbours are touched. Repeat along the affected wall and the footing ends up sitting on a row of concrete blocks founded on better ground.

Why the sequence matters so much

A brick wall can bridge over a short unsupported gap; it cannot bridge over a long one. That is the entire logic of pinning. The engineer's drawings will show a hit-and-miss sequence — pin one, then pin four, then pin two — so that no two adjacent sections are ever open at the same time and the footing is never undermined beyond what the wall above can safely span.

This is where cheap operators cut corners. Digging three pins at once saves days, and it works fine right up until it doesn't. Ask any contractor to walk you through their pin sequence and how long each pin cures before it takes load. A good one answers without blinking, because the answer is on their engineer's drawings — our guide to engineer's reports explains what that design work involves.

Where it wins, and where it doesn't

Mass concrete earns its keep when sound bearing soil sits at modest depth, roughly within 1.5 m of the existing footing. In that scenario it is hard to beat. The method is more than a century old and thoroughly proven, it needs no specialist plant, any competent underpinning contractor can price it accurately, and the finished product is solid, inert concrete with nothing to corrode and nothing to argue about later.

Its weaknesses are practical. Every pin is dug largely by hand, so there is real disruption: open trenches along the wall, spoil to cart away, garden beds and paths sacrificed, and weeks of work rather than days. It needs access too — a footing hard against a boundary fence or under a low deck is slow and expensive to reach. And if good soil sits 3 m down rather than 1 m, hand excavation becomes uneconomic and unsafe; that is screw pile territory. For shallow re-compaction problems under a slab, resin injection may be the cleaner comparison. Our methods overview puts all three side by side.

Cost and timeline

Expect roughly $2,000–$5,000 per metre of underpinned footing, driven by depth, soil and access. A typical settling corner needs 3–6 m of underpinning, so most corner jobs land between about $8,000 and $20,000, stretching towards $30,000 where pins are deep and access is poor. That sits inside the $10,000–$40,000 band that covers most residential underpinning — our cost guide shows how quotes are built up.

Allow 2–4 weeks on site for a typical residential job, because each pin must cure before the next begins. Engineering, soil testing and any approvals come before that; the process guide walks through the full sequence.

What to check in a quote

A mass concrete quote should be specific enough to compare line by line. Look for:

• The metres of footing being underpinned and the design depth of each pin
• A reference to the engineer's design, including the pin sequence and cure times
• Whether spoil removal and reinstatement of paths and gardens are included or excluded
• How the dry-pack connection to the existing footing will be done
• Warranty terms and current insurance details